The Role of CaBP1 and 2 in Hair Cell Synaptic Function

In mammalian inner hair cells (IHC), voltage-gated Calcium channels CaV1.3, which show remarkably little inactivation, are driving presynaptic calcium influx. CaBPs might play an important role for IHC CaV1.3 modulation, as they bind to voltage-gated calcium channels and reduce inactivation of depolarization-evoked calcium currents in various cell types. In IHCs, CaBP family members 1, 2, 4 and 5 are co-expressed, but, so far, only mutations in CaBP2 were found to lead to hearing impairment in humans (DFNB93), which is characterized by moderate to severe low- to mid-frequency hearing loss. Up to now, we do not understand the contribution of the different CaBPs in the regulation of CaV1.3 channel properties in IHCs. We investigated the role of different CaBPs at the first auditory synapse employing system physiology (ABR), in vitro patch-clamp of IHCs and immunohistochemistry in CaBP1 and 2 double knockout (DKO) mice. With the patch-clamp technique, we assessed the presynaptic function of calcium and barium currents and measured exocytosis though membrane capacitance changes. Inactivation of Ba²⁺ and Ca²⁺ currents was strongly enhanced, suggesting increased voltage- and calcium-dependent inactivation of CaV1.3 (VDI and CDI, respectively). This resulted in significantly reduced exocytosis. Immunohistochemistry of IHC synapses revealed enlarged postsynaptic immunofluorescent signals suggesting modulation of postsynaptic density by presynaptic IHC activity.

CaBP1, 2 and 4 together modulate CDI of CaV1.3 channels in IHCs. These effects are partially obscured in single KOs due to compensation by other CaBPs. CaBP2 differs from other CaBPs in its ability to further modulate VDI of CaV1.3, which is critical for IHC function and hearing.

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Publication History

Article published online:
10 June 2020